1. Field of the Invention
The present invention relates to a light-emitting device having a light emitting element using a high molecular weight organic compound (high molecular weight material). In particular, the present invention relates to a light emitting device in which an organic compound (polymer) composed of the same repetition units partly includes a phosphor and a manufacturing method thereof. Note that a light-emitting device in this specification indicates an image display device, a light-emitting device or a light source. Also, a module in which a connector, for example, an FPC (flexible printed circuit), a TAB (tape automated bonding) tape, or a TCP (tape carrier package) is attached to the light emitting element, a module in which a printed wiring board is provided in the end of the TAB tape or the TCP, and a module in which an IC (integrated circuit) is directly mounted on the light emitting element by a COG (chip on glass) method are included all in the light-emitting device.
2. Description of the Related Art
A light emitting element in the present invention is an element for emitting light by applying an electric field thereto. With respect to the light emitting mechanism, it is said that an electron injected from a cathode and a hole injected from an anode are recombined in an organic compound layer by applying a voltage to electrodes sandwiching an organic compound layer to produce a molecule with an excitation state (hereinafter referred to as “a molecular exciton”) and the molecular exciton releases energy to emit light when it is returned to a ground state.
Note that, although it is considered that a singlet excitation state and a triplet excitation state are possible with respect to a kind of molecular exciton produced by the organic compound, either excitation state is included in this specification when it contributes to light emission.
In such a light emitting element, the organic compound layer is generally made from a thin film having a thickness less than 1 μm. In addition, since the light emitting element is a self-luminous type element such that the organic compound layer itself emits light, a back light used in a conventional liquid crystal display is not required. Thus, it is the big advantage that an extremely thin and lightweight light emitting element can be manufactured.
Also, when the carrier mobility of, for example, an organic compound layer having a thickness of about 100 nm to 200 nm is considered, a period from the injection of a carrier to the recombination is about several ten nanoseconds. Even when a period required for a process from the recombination of a carrier to light emission is included in the period, light emission is conducted within the order of microsecond. Thus, an extremely high response speed is one of characteristics thereof.
Further, since the light emitting element is a carrier injection type light emitting element, it can be driven by a direct current voltage and a noise is hard to generate. With respect to a drive voltage, the organic compound layer is made from a uniform ultra thin film having a thickness of about 100 nm, an electrode material such that a carrier injection barrier to the organic compound layer is decreased is selected, and a hetero structure (two-layers structure) is introduced. Thus, a sufficient luminance of 100 cd/m2 at 5.5 V has been achieved (Reference 1: C. W. Tang and S. A. VanSlyke, “Organic electroluminescent diodes” Applied Physics Letters, vol. 51, No. 12, pp. 913–915 (1987)).
From characteristics such as a thin type, lightweight, high speed responsibility, and direct-current low-voltage drive, the light emitting element has been noted as a next generation flat panel display element. In addition, since the light emitting element is a self-luminous type and has a wide viewing angle, the visibility is relatively good. Thus, it is considered that the light emitting element is effective as an element used for a display screen of a portable device.
Now, a material composing the organic compound layer is broadly categorized into two materials: a low molecular weight material and a high molecular weight material.
If comparisons are conducted from the film formation aspect, those are different in the following point. That is, although the low molecular weight material is formed as a thin film on a substrate by a vacuum evaporation method, the high molecular weight material is formed from a solution of an organic solvent on a substrate by a wet process such as spin coating. Note that, when the film formation is conducted by the vacuum evaporation method, conventional patterning using a shadow mask technique is possible. In addition, there is an advantage that the purity of a material can be kept because of a dry process in a vacuum. On the other hand, in the case of the spin coating method, there is an advantage that the film formation on a large area substrate is easy and can be conducted for a short time at a low cost. Therefore, the development of an element capitalizing respective characteristics has been progressed.
However, when it is considered from the material aspect, it is an advantage that the high molecular weight material having physical properties such that a mechanical strength is large and cohesion and crystallization are hard to produce is used. When the high molecular weight material has a large mechanical strength, a flexible element capable of tensile and bending processings can be manufactured. When cohesion and crystallization are hard to produce, a use under a high temperature condition can be expected. In other words, when the high molecular weight material is used, a condition for manufacturing an element is relaxed and the variety of use can be expected.
Also, a light emitting region including a phosphor is formed in a portion of an organic compound layer to set a recombination region of carrier. Thus, the diffusion of carrier is prevented, the recombination region can be distanced from electrodes, and a light emitting characteristic is improved by two times (Reference 2: C. W. Tang, S. A. VanSlyke, and C. H. Chen; Electro Luminescence of Doped Organic Thin Films; J. Appl. Phys., vol. 55, No. 9, pp. 3510–3515 (1987)).
Also, although it is different from the case where a phosphor is included in a portion of the organic compound layer of a light emitting element using a high molecular weight material and the light emitting region is formed therein, a laminate structure as shown in FIG. 11 has been well known.
In FIG. 11, an anode 1102, an organic compound layer 1103, and a cathode 1104 are formed on a substrate 1101. The organic compound layer 1103 becomes a laminate structure of a hole transport layer 1105 and a light emitting layer 1106. Here, the hole transport layer 1105 is made of PEDOT (poly(3,4-ethylene dioxythiophene)) and polystyrene sulfonic acid (PSS) as an acceptor material and the light emitting layer 1106 is made of a copolymer of polyparaphenylene vinylene. Note that, since a material having different solubility to a solvent is used for each of these layers, a laminate structure can be formed.
However, the report in which the light emitting region including a phosphor is formed in a portion of the organic compound layer to set a recombination region of carrier is made in the case where the low molecular weight material is used. When the high molecular weight material is used, there is a problem that the formation of the light emitting region partly including a phosphor is difficult from the material characteristic.
Also, in the case of the laminate structure as shown in FIG. 11, a functional separation by the laminate structure using the high molecular weight material is possible. On the other hand, since a material composing the hole transport layer 1105 is different from that composing the light emitting layer 1106, there is also a problem that quenching due to energy transfer in the laminate interface therebetween is easy to cause.